A boat drive with a standard variable- or adjustable-pitch propeller is described in U.S. Pat. No. 5,073,134. It has a drive housing projecting from a hull of a boat, a tube shaft journaled in the housing and centered on a main axis, and a drive for rotating the tube shaft. A propeller hub mounted on the propeller shaft is provided with a plurality of adjustable-pitch propeller blades rotatable about respective axes generally radial of the main axis. Respective blade-adjustment rods extending parallel to the propeller shaft are shiftable axially to adjust a pitch of the blades and the hub has an end turned toward the housing and a free end turned away from the housing. A pitch-adjustment push rod passing through the propeller shaft and projecting out of the hub with one extremity of the push rod at the free end of the hub is axially shiftable in the propeller shaft by a double-acting piston-and-cylinder unit in the housing connected to the opposite extremity of the push rod. Another such system is described in German 3,118,230.
In German patent 878,906 of Wels the core shaft is axially fixed in the tube shaft and pivotal to change the setting of the propeller vanes. A worm gear meshing with a gear wheel on the core shaft is used to effect the desired rotation.
German patent 1,065,339 of Fischer has an axially displaceable core shaft whose front end is threaded in an axially fixed nut that can be coupled to the tube shaft for joint rotation therewith or coupled to the housing so that, when the tube shaft rotates relative to the housing, the screw moves axially in the nut and changes the vane setting.
These systems are all quite complex. Those with a core shaft normally rotating relative to the tube shaft are subject to considerable wear. The hydraulic arrangements often must be pressurized at high pressure to maintain a vane setting, and the all systems are quite complex.